1. Field of the Invention
The present invention relates to an electroluminescent display device (hereinafter referred to as an xe2x80x9cEL display devicexe2x80x9d) and more particularly to a large surface-area EL display device.
2. Description of Related Art
With the recent advent of diversified information processing units, there is an increasing need for flat display elements which consume less electricity and have less weight compared to generally-employed CRTs. An EL element has received notice as one type of such flat display elements. The EL elements are generally classified into two categories depending upon the materials used: inorganic EL elements and organic EL elements.
The inorganic EL elements include a luminescent portion which emits light when its luminous centers are excited by bombardment of electrons accelerated in the intense electric field applied.
On the other hand, the organic EL elements utilize the fluorescent emission of an organic molecule that occurs when it returns toga ground state from an excited state upon recombination of an electron from an electron injecting electrode with a hole from a hole injecting electrode at its luminous center.
If the inorganic EL elements are to be driven, a high voltage in the range of 100-200 V must be applied to produce an intense electric field. In contrast, the organic EL elements can be driven at a lower voltage in the approximate range of 5-20 V. Another advantage is that the suitable selection of luminophors results in different color emissions of the organic EL elements, which makes them suitable for use in multi-color and full-color display devices.
These EL elements have excellent features distinguished from liquid crystal displays, such as spontaneous emissions, independence of view angle and thickness reduction resulting from elimination of a back light, and their applications on large surface-area display devices have been energetically researched and developed.
A variety of modes have been investigated for driving display devices utilizing such EL elements, as similar with the case of liquid crystal displays. A driving mode utilizing a TFT or other switching element positioned in each pixel has gained attention for its higher contrast ratio and middle tone characteristics. In the case of organic EL elements which are current-driven light emitting elements, a current of about 20-30 mA must be supplied to each pixel. It is difficult to realize such a current level with the use of a cell array or driver circuit constructed using amorphous silicon TFT. This necessitates a cell array or driver circuit constructed using polycrystalline silicon TFT. Silicon, when deposited on a glass substrate or the like, must be crystalized at a low temperature. That is, the use of low-temperature polycrystalline silicon TFT as a switching element becomes necessary.
It is however difficult to deposit, in the form of a uniform film, the low-temperature polycrystalline silicon on a large surface-area substrate. This has been one of barriers to area expansion of organic EL display devices.
A first object of the present invention is to provide an EL display device, such as an organic EL display device, which has a sufficient structural strength and can be fabricated in a large size.
A second object of the present invention is to provide a process for fabrication of an EL display device, which enables size increase of EL display devices if incorporating circuits (e.g., switching circuits) that are unsuited for large surface-area formation.
The EL display device in accordance with a first aspect of the present invention is characterized as being increased in size by securing plural small-size panels, each carrying plural EL elements on its substrate, to a large-size support by an adhesive layer which faces toward those EL elements.
The EL elements are known as having a tendency to deteriorate when exposed to moisture, and thus require a sealing metallic form, called a can, or the like which is attached in such a fashion to seal them from an ambient environment. In the first aspect, plural small-size panels are secured to a large-size support by an adhesive layer which faces toward those EL elements. This measure used to increase sizes of EL display devices serves not only as a sealing means for preventing the aforementioned deterioration but also as a reinforcing means for sustaining the strength of the display devices. Therefore, the first aspect simultaneously achieves sealing of the EL elements, reinforcement and size increase of the display devices.
In the first aspect, a transparent substrate, such as a glass substrate, can be used as a substrate for the small-size panel. In such an instance, the glass substrate may define a light-exiting side of the device, so that a light emitted from the EL element exits the device through the glass substrate. The large-size support facing toward the EL elements is then located opposite to the light-exiting side of the device.
In general, an electrode of the EL element placed on the light exiting side is a transparent electrode such as of ITO. Another electrode such as of a metal film is placed on the opposite side. This metal film electrode is opaque and reflects a light in a manner unique to a metal surface. Accordingly, the region where there is a metal electrode provides a different appearance from the region where there is no metal film electrode. Such an uneven appearance can be improved with the use of a large-size support made of a metal or having a metallic film laminated on its attachment side, whereby a metal surface is provided even in regions where there is no metal film electrode. Particularly, the presence of such a metallic support or metallic film at gaps created between neighboring small-size panels reduces the difference in appearance of the joints between small size panels from the remaining regions.
In the first aspect, the large-size support may be made of transparent material. The use of large-size transparent support permits the involvement of an ultraviolet-curable adhesive layer. That is, an adhesive layer, prior to being cured, may be interposed between the plural small-size panels and the large-size transparent support to join them. The subsequent exposure of the adhesive layer, through the large-size transparent support, to an ultraviolet light source causes it to cure.
In the case where the large-size transparent support is employed, such a large-size transparent support may be located at the side of the device from which a light emitted from the EL element exits. That is, the ITO or other transparent electrode defining an uppermost layer of the EL element may be located to face the large-size transparent support so that a light from the EL element is caused to exit the device through the large-size transparent support.
It is generally preferred that the adhesive layer be provided to overlie an entire area of an attachment surface. However, in the case where the large-size support is located opposite to the light-exiting side of the device, the adhesive layer may be provided to overlie only a periphery of each small-size panel. In such a case, the peripherally-extending adhesive layer defines an interior space between the small-size panel and large-size support. The placement of a desiccant in such a space effectively reduces deterioration of the EL element that may be caused by the exposure to a moisture.
The EL display device in accordance with a second aspect of the present invention is characterized as being increased in size by securing plural small-size panels, each carrying plural EL elements on its substrate, to a large-size support by an adhesive layer which faces toward the substrate of each panel.
In the second aspect, the securement of plural small-size panels to the large-size support by the adhesive layer imparts sufficient structural strength to a resulting large surface-area display device. A variety of techniques exist for sealing the EL elements to thereby suppress their deterioration. For example, a sealing metal form, called a can, or the like may be attached in such a fashion to seal the EL elements from an ambient environment. Alternatively, a sealing substrate such as a glass substrate may be attached with the aid of a sealing resin to seal them. Other useful sealing techniques include deposition of a film of a sealing resin on the EL elements and securement of a sealing film to overlie the EL elements.
In the second aspect, a glass or other transparent substrate may be used for the substrate of each small-size panel. In such an instance, the display device may be configured such that a light emitted from the EL element exits through such a transparent substrate. In the case where a light emitted from the EL element is caused to exit the device through the transparent substrate, the large-size support secured in contact with the transparent substrate of each panel may also be rendered transparent such that a light emitted from the EL element exits the device through the transparent substrate and the large-size transparent support. In this case, the large-size transparent support preferably has a refractive index comparable in level to the transparent substrate of the small-size panel, i.e., within xc2x11% deviation from the refractive index of the transparent substrate.
In the second aspect, an antireflection film may be used for the large-size support. Such an antireflection film has been conventionally employed when a metal film defines a pixel electrode to reduce its reflection and thereby increase contrasts. The use of antireflection film for the large-size support thus simplifies a construction and configuration of the EL display device.
In the first and second aspects of the present invention, it is preferred that the neighboring small-size panels are additionally secured to each other at their opposing faces by an adhesive layer. Such adhesion of small-size panels at their opposing faces may be achieved either before or during they are secured to the large-size support.
Preferably, the adhesive layer interposed between the opposing faces of small-size panels include spacers. The inclusion of such spacers permits suitable adjustment of a distance between neighboring small-size panels, so that a spacing between adjacent two electrodes within one small-size panel can be adjusted equal in dimension to a spacing between adjacent two electrodes separated into neighboring small-size panels.
In accordance with the first and second aspects of the present invention, the small-size panel carrying the EL elements may also incorporates driving circuits for driving them or connections with an external driving circuit. Such construction permits the EL elements within one small-size panel to be driven via the driving circuits or connections provided in the same small-size panel and thus eliminates the need to provide interpanel connections for coupling scanning pixel electrodes and/or signal pixel electrodes between different small-size panels, thereby facilitating the size increase of a display device.
The method for fabrication of an EL display device in accordance with the present invention is characterized as comprising, in sequence, securing plural small-size panels, each carrying a circuit in each pixel region, to a large-size support, forming an electroluminescent element over the pixel region, and sealing every electroluminescent element.
In accordance with the present invention, plural small-size panels, each carrying a circuit (e.g., a TFT or other switching element) in each pixel region, are secured to a large-size support, resulting in the provision of a large-size display panel. This assures that a consistent quality of circuit can be formed in each pixel region of the small-size panel. The occurrence of circuit defects and other inconveniences can be markedly reduced compared to the case where all necessary circuits are directly formed on a large-size substrate.
In accordance with the present invention, the circuit formation can be accomplished over a small-size panel. This allows the use of low-temperature polycrystalline silicon prepared by crystalizing amorphous silicon, for example, in the formation of a TFT or other switching element circuit, and is particularly suited to the case where amorphous silicon is crystalized by laser annealing.
In the present invention, it is preferred to plug a gap, if formed between small-size panels subsequent to their securement to the large-size support. The provision of such a plug portion prevents the passage of a moisture into the sealed interior so that the deterioration of EL elements by the moisture is effectively prevented.
In the present invention, subsequent to securement of small-size panels to the large-size support, the EL elements are formed and then all sealed. While not particularly specified, sealing may be accomplished by attachment of a sealing member. In such an instance, a seat portion having an upper surface contiguous in level with a top surface of the aforementioned plug portion may preferably be provided to bear the sealing member.
Examples of useful sealing members include metallic forms as of aluminum and stainless steel, inorganic ceramic forms as of SiO2 and Al2O3, and hydrophobic forms containing plastics or the like.
The aforementioned plug and seat portions can be formed of a negative photosensitive material by a photolithographic technique. Preferably, a passivation film may be provided to overlie such plug and seat portions to prevent passage of a moisture therethrough. Examples of useful passivation films include SiO2, SiN, Al2O3 and AlN films. These films can be formed by a sputtering technique, for example.
Also, the provision of driving circuits for driving switching elements or connections with an external driving circuit in the common small-size panel eliminates the need to provide interpanel connections for electrically coupling pixel electrodes between separate small-size panels. As a result, the size increase of EL display devices can be accomplished in a more simplified fashion.